1. Field of the Invention
This invention relates generally to the field of air filter canisters having particulate filter elements such as those used in conjunction with mask assemblies (e.g. industrial respirators) and the like. More particularly, this invention relates to an improved method and apparatus for sealing such filter elements to the walls of such filter canisters.
2. Background of the Invention
Air filter canisters are used in a wide number of industrial applications to provide protection against hazardous gases, vapors, aerosols, dusts and fumes. Many other environments also use or could use such filters, including military applications. Such filters commonly include one or several filter mechanisms. Typically such filters include a pleated paper filter element used to provide a large surface area of particulate filtering with minimal resistance to air flow and long life before the filter becomes clogged. The particulate filter may be used in conjunction with a charge of activated charcoal which may be doped with any of a variety of complexes or compounds to enhance its ability to adsorb or react with harmful vapors.
It is, of course, essential for such canisters to have a good seal at the periphery of the paper element adjacent the canister housing so that all of the air passing through the filter canister is forced to pass through the paper filter element (as well as other elements of the filter) to ensure maximum effectiveness of the filter. Otherwise, air (or whatever fluid is to be filtered) will pass through any gaps in the seal and the filter's effectiveness will be compromised.
Several matters must be taken into consideration when devising a sealing scheme for such filters. Since such filters are only useful for a limited time when exposed to the environment or use, they are produced and used in large quantities. Accordingly, production cost should be minimized and the time which any charcoal charge is exposed to the atmosphere during the production process should be minimized. Any liquid sealant which is used should, to the greatest extent possible, be limited to contacting the edge of the paper element (rather than the surface) so that the surface area of the particulate filter element is not reduced any more than necessary.
In a prior method, the paper filter was encapsulated into a metal cup-like retainer with air passages to form a subassembly. The sub-assembly was produced by placing the pleated paper filter into the cup-like retainer and placing that into a mold. A large metal disk which is slightly smaller in diameter than the filter element is placed on top of the filter element and then the assembly is spun. While it is spinning, an injection of urethane, or other sealant, is applied to the spinning disk.
The spinning disk distributes the urethane to the edge of the disk to mold the urethane to the shape of the mold into which the filter medium and retainer were placed. The disk is then removed and the sub-assembly is removed from the mold. This subassembly is then inserted into the canister body and sealed around its circumference with a bead of liquid sealant such as RTV silicone sealant in a secondary operation. This sealant is difficult to apply consistently, requires close supervision and adjustment of the bead of sealant, and requires an elaborate cure cycle. In addition, this process, for production requires use of numerous molds. The net result is a higher cost canister with greater possibility of increased scrap rates in production.
Filter canisters of the type generally related to the present invention are available commercially. Examples are the models P3 and PM3/TPM3/SP3 available from Racal Filter Technologies, LTD., 1175 California Ave., P.O. Box 665, Brockville, Ontario, Canada K6V 5V8. Similar styles of canisters are shown and described in, for example, U.S. Pat. No. 4,714,486 to Silverthorn as well as U.K. Patent Application No. 2,223,423 and European Patent Application No. 89202311.0 (Publication No. 0 362 920) both to Meunier which are hereby incorporated by reference. The Silverthorn reference briefly describes a polyurethane seal which is produced by spinning the canister so that the seal is forced to the edge of the paper element by centrifugal force. The U.K. application describes a compression seal arrangement.
Another spinning technique is disclosed in U.S. Pat. No. 3,389,031 (Re. 27,466) and U.S. Pat. No. 3,466,413 to Rosaen et al. in which a pleated filter is placed vertically inside a mold (pleats running side to side rather than top to bottom). The sealant is then poured into an opening in the mold while the mold is spun. Similar filter canisters are shown in U. K. Patent Application No. 2,140,310 to Ansite and U. K. Patent No. 1,247,524 to Etat Francais. Other filter sealing techniques are shown in U. K. Patent No. 1,318,773 to Starkie, U. K. Patent No. 1,446,195 to Carena, U. K. Patent No. 1,553,926 to Texas Research Institute, Inc. and U.S. Pat. No. 4,278,455.
The present invention, in its preferred embodiment, provides an improved method and apparatus for sealing a filter canister in a cost effective manner which minimizes reduction of paper pleated filter contamination with sealant. In addition, the present invention provides a mechanism for sealing the canister after the top has been applied without significantly affecting the flow resistance.